A specific, irreversible and cost-effective topical microbicide for HIV prophylaxis will help slow sexual transmission of the pandemic. Here, we propose the development of catalytic antibodies capable of degrading the HIV envelope protein gp120 as candidate HIVcides. The promising features of the antibodies are the permanent destruction of the envelope protein, reuse of a single catalyst molecule to degrade thousands of gp120 molecules and the recognition of a conserved gp120 region, the superantigen region, permitting neutralization of diverse HIV strains. Polyclonal antibody studies from uninfected and HIV infected subjects indicated that high level proteolytic and HIV neutralizing activities is a noteworthy property of IgA class antibodies. In the R21 project phase, characterization of our existing antibodies and their single chain Fv (scFv), IgA and IgG variants is proposed. The existing antibodies were obtained as IgGs by immunization with an electrophilic gp120 analog that induces antibodies with enhanced nucleophilic reactivity, a prerequisite for the catalytic reaction. Using an electrophilic probe to the gp120 superantigen site, additional proteolytic antibodies were obtained as scFv constructs cloned from the immune repertoire of lupus patients, who tend to produce antibodies with proteolytic activity directed to the gp120 superantigen site. The antibodies will be characterized with respect to HIV degrading efficiency; specificity, potency and breadth of HIV neutralization; and the ability to perform these functions in the vaginal milieu. To isolate novel, improved antibodies, we will screen the proteolytic and HIV neutralizing activity of salivary and serum IgAs from HIV-negative and HIV-positive subjects in the R21 project phase. In the R33 phase, monoclonal IgAs with the desired properties will be cloned from lymphocytes by cell-fractionation based on covalent binding of an electrophilic gp120 peptide analog, a property associated with proteolytic antibody-producing cells. Proof-of-principle for in vivo antibody efficacy will be obtained in the R33 phase using the SHIV-macaque model of infection. The Abs will also be examined for activity in vitro as microbicide excipient formulations under conditions simulating the vaginal environment following sexual intercourse. These studies may identify proteolytic antibodies suitable for further development as a topical HIVcide. [unreadable] [unreadable] [unreadable]